Time signal
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A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.
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Audible and visible time signals
One sort of public time signal is, of course, a striking clock. These clocks, however, are only as good as the clockwork that activates them; they have improved substantially since the first surviving clocks from the fourteenth century. For many members of the general public, a public clock such as Big Ben was the only time standard they needed.
When more accurate public time signals were desired for use in navigation, a number of traditional audible or visible time signals were established for the purpose of allowing navigators to set their chronometers by. These public time signals were formerly established in may seaport cities.
As an example of such a signal, in Vancouver, British Columbia, a "9 o'clock gun" is still shot every night at 9 PM. This gun was brought to Stanley Park by the Department of Fisheries in 1894 to warn fishermen of the 6:00 PM Sunday closing of fishing. The 9:00 PM firing was later established as a time signal for the general population. The Brockton Point lighthouse keeper, William D. Jones, originally detonated a stick of dynamite until the Time Gun was installed. A similar "noon gun" was formerly shot every noon at Cape Town, South Africa. A cannon was shot at one o'clock every weekday at Liverpool, England, at the Castle in Edinburgh, Scotland, and also at Perth in Australia to establish the time.
The ceremony of "dropping the ball" at New Year's Eve in Times Square in New York City is a vestige of a visual indication of time. The first such time ball was installed in 1833 on the roof of the Royal Observatory in Greenwich, England. The ball was set to drop at 1 P.M. to enable the astronomers to establish the correct moment it should drop by observing the sun at noon. Liverpool, Edinburgh, and Perth also had balls that were dropped concurrently with the firing of the time cannons. Because the speed of light is much faster than the speed of sound, visible signals enabled greater precision than audible ones; however, the audible signals could operate under conditions of reduced visibility. In 1861 and 1862, the Post Office Directory had time gun maps published that related the number of seconds it took for the report of the time gun to reach various locations in Edinburgh.
In many non-seafaring communities, loud factory whistles served as public time signals before radio made them obsolete.
Electrical and electronic time signals
Standard time came into existence in the United States on November 18, 1883. Earlier, on October 11, 1883, the General Time Convention, forerunner to the American Railway Association, approved a plan that divided the United States into several time zones. On that November day, the United States Naval Observatory telegraphed a signal that coordinated noon at Eastern standard time with 11 AM Central, 10 AM Mountain, and 9 AM Pacific standard time.
A March, 1905 issue of The Techical World describes the role of the United States Naval Observatory as a source of time signals:
- One of the most important functions of the Naval Observatory is found in the daily distribution of the correct time to every portion of the United States. This is effected by means of telegraphic signals, which are sent out from Washington at noon daily, except Sundays. The original object of this time service was to furnish mariners in the seaboard cities with the means of regulating their chronometers; but, like many another governmental activity, its scope has gradually broadened until it has become of general usefulness. The electrical impulse which goes forth from the Observatory at noon each day, now sets or regulates automatically more than 70,000 clocks located in all parts of the United States, and also serves, in each of the larger cities of the country, to release a time-ball located on some lofty building of central location. The dropping of the time-ball — accompanied, at some points, with the simultaneous firing of a cannon — is the signal for the regulation by hand of hundreds of other clocks and watches in the vicinity.
This telegraphic distribution of time signals has been obsoleted by the use of radio.
Time signal sources that can be used as references for radio clocks include:
- the WWV, WWVB and WWVH radio stations in the United States
- the CHU radio station in Canada
- the DCF77 radio station in Germany
- the MSF radio station in the United Kingdom
All of these stations now broadcast a time code.
Loran-C time signals may also be used for radio clock synchronization, by augmenting their highly accurate frequency transmissions with external measurements of the offsets of LORAN navigation signals against time standards.
The Global Positioning System can also be used as a time reference for radio clocks.
Many people whose needs do not require precision to the second simply rely on radio broadcasting for time signals. The British Broadcasting Company famously broadcasts "pips" and the strike of Big Ben at the hour in its broadcasts.
See also
- Time transfer
- Network time protocol
- Speaking clock
- Synchronization
- atomic clock
- Greenwich Time Signal
External links
- Insight into marine science (http://www.pol.ac.uk/home/insight/timeball.html): discusses the use of time balls in maritime cities
- Travel Downunder (http://www.traveldownunder.com.au/Western_Australia/Perth/Round_House_Precinct,_The.asp) mentions the time cannon at Perth
- Durr Cannon Research Projects (http://www.museums.org.za/cannon/prod02.htm#Noon%20Gun): mentions the time cannon at Cape Town
- The Edinburgh time gun (http://www.edinphoto.org.uk/1_edin/1_edinburgh_history_-_time-gun.htm)
- Distribution of Time Signals (http://earlyradiohistory.us/1905tim.htm) by the U.S. Naval Observatory
Reference
- Downing, Michael, Spring Forward: The Annual Madness of Daylight Saving Time (Shoemaker and Hoard, 2005) ISBN 1-59376-053-1